The background description provided herein is for the purpose of generally presenting the context of the disclosure and may not constitute prior art.
Conventional light-water reactors (LWRs) operate on a once-through cycle by burning uranium enriched in a rare isotope U-235 relative to the more abundant U-238. The LWRs use water as coolant to carry away the heat of the fission reactions. This arrangement, however, (1) uses only about 1% of fuel potentially available in natural uranium; (2) generates plutonium and other higher actinides, primarily Pu-239 from neutron capture on U-238 plus two beta decays, which creates a risk for proliferation of nuclear weapons; and (3) produces high-level waste that is difficult to dispose of because of an awkward mixture of short-lived radioactivities (30-year and less) and long-lived radioactivities (24,000 years and more).
Proposals have been made to convert from a “uranium economy” to a “plutonium economy” through the separation of fission products from the actinides for easier disposal, recycling of the plutonium and minor actinides as fuel to replace the once-through cycle, and enhanced breeding of the fissile Pu-239 from the fertile U-238 using solid fueled fast breeders cooled by liquid metals. These proposals, however, raise environmental, safety, and security concerns, as well as issues of high financial costs.
Molten-salt reactors (MSRs) offer an attractive alternative to the nuclear option for power generation. Because the fuel as well as the coolant is in a liquid rather than a solid state, chemical separation of fission products from fissiles and fertiles can be done on-site to achieve virtually 100% burn-up. This obviates multiple back-and-forth transports of nuclear materials from nuclear power plants to reprocessing and prefabrication centers that otherwise occur with the conventional recycle strategy. Moreover, substituting Th-232 for U-238 as the fertile stock for breeding into U-233 rather than Pu-239 simplifies the problem of disposal of high-level nuclear waste, thereby greatly reducing both volume and time that such material needs to be stored in a repository before radiation drops below background levels.
Neutron irradiation of Th-232 produces virtually no plutonium or heavier actinides, closing that particular path to proliferation of nuclear weapons. Moreover, if Th-232 is uninterruptedly irradiated over a period of one year or longer, there is sufficient U-232 generated along with the U-233 as to create a strong deterrent against nuclear weapons production. As part of its decay chain, U-232 has strong gamma emission that makes detection of its presence easy. Additionally, the strong gamma emission interferes with any electronic mechanisms associated with a weapon.
The MSRs also have features that promote inherent passive safety. For example, the features include (1) thermal expansion of fuel out of a reactor core when fission reactions run too fast; (2) drainage of fuel into sub-critical holding tanks through melting of freeze plugs if the fuel salt gets extremely hot; (3) inward instead of outward blowing of leaking radioactive gases because of low vapor pressure of molten salts; and (4) immobilization of radioactive fuel or blanket through solidification of the molten salts if they should somehow escape from their primary or secondary containment vessels or piping.
The MSRs, however, may raise issues of chemical corrosion of containers and separation of fission products from fissile and fertile elements dissolved in the fuel and blanket salts. Because many of the fission products from the former behave similarly to thorium in the latter, a simple approach is to keep separate the uranium/plutonium fuel and thorium breed-stock in a two-fluid scheme as part of the overall reactor design.
Early systems used single fluid designs that are graphite moderated. Recent studies indicate that such designs may be unstable to unanticipated large excursions to high temperatures that enhance the capture of neutrons by U-233 relative to those by Th-232. Two-fluid MSRs, however, present significantly greater challenges for “plumbing” than single-fluid designs. This tension between chemical complexity/simplicity and plumbing simplicity/complexity has existed since the earliest discussions on building of a molten-salt breeder reactor.